Feature Request: FirstTimeReady timestamp(unixtimestamp) as gauge metrics.

[dineshkumar181094]

Reason - we wanted to measure how fast our node is ready for scheduling.
This can be calculated by metrics from kube state metrics (unix timestamp) kube_node_created.
If we get metrics from taint controller about at what time(unix timestamp) node is marked ready.

we can make this calculation by subtracting those metrics.

[dineshkumar181094]

I can also work on this if this make sense, or there is some other work around let me konw.

[dineshkumar181094]

@Joseph-Irving what do you think is it worthy?